Trajectories for Principal Stresses 1

[releky]

This is the trajectories for principal stress for uniform load

http://imagizer.imageshack.us/v2/800x600q90/20/jyq8.jpg

I'm looking for illustration of trajectories of principal stress for concentrated load at midspan. Has anyone encountered this?

 

[BAretired]

releky,

No, 500mm would not be shallow either, but I was under the impression that we were talking about tee beams. The depth of a tee beam is from the underside of beam to top of slab. Are we talking about the beams in thread507-357341? If so, the depth is 600mm.

BA

 

[BAretired]

Coming from an area where seismic design is not a serious requirement, I am not exactly sure how that would affect the beam size and reinforcement, but from the standpoint of gravity loading, the beam sizes look reasonable to me.

BA

 

[releky]

Yes. The same beam. From bottom of beam to top of slab. It's 500mm.. so the entire depth of the beam is 500mm. If just the underside of the visible beam of a T-beam, then it's only 400mm visible. Of course when we specify a beam depth. We are talking of the entire depth of the beam from bottom to top surface of slab (100mm slab). The top bars of the beam is in the level of the middle of slab. So the slab flange is part of the beam.. unless you lower the bars to below the slab which would make the effective depth of the beam shallower. But this isn't what happened. So it's officially 500mm which includes the 100mm T-beam flange.

In moment frame, you have to multiply the U = 1.2 DL + 1.6 LL by two to design the longitudinal bars and shear stirrups. All the longitudinal bars of the buildings are design for 2 x (1.2 DL + 1.6 LL), only the stirrups is designed just for gravity load due to the designer not bothering to check the shear and moment diagram of the ETABs outputs.

Now they said they would try to put I-beam to improve the shear resistance but don't know how to start.

 

[BAretired]

releky,
Here is the sketch shown in the previous thread. The depth looks like 600mm to me. How can that be interpreted another way?

http://img571.imageshack.us/img571/1747/fb1m.jpg

BA

 

[BAretired]

Okay, just realized how it could be interpreted another way. I thought the 19.68" dimension was the depth of beam below the slab. You meant it to be the overall depth. Sorry about that. But still, the depth seems to be in the ballpark to me.

BA

 

[releky]

AELLC and BA,

In the Philippines.. engineers are free from liability in case building is damaged by any seismic activity so designers don't worry about damages caused by earthquake because they can always blame it on the earthquakes. So in a country where 99% of buildings are special moment frames. They are just ordinary moment frames or even just gravity frames.

BA, seismic design is simply making sure the sections have adequate shear strength before developing plastic hinges. To develope plastic hinges, you must exceed up all flexural reserve at factored load. So the shear must be as strong as that able to develope the so called "probable moment strength".

AELLC. Even without peer review, the shear is only design for gravity load... not seismic loading. When you say deflections from crack section.. are you referring to flexural cracks? I'm not worried about flexural cracks because its flexural design is adequate up to probable moments srength. I'm more worried about shear cracks of shallow beams. Have you ever encountered any such shear cracks in your life?

 

[RFreund]

First I'd like to applaud your efforts. Second could you maybe summarize your concerns in a bulleted list type format and see if we can put this to bed.

You seem to show a calculation that shows the beam is OK for gravity load but then you say you are worried about gravity load. You say you're worried about stirrups at midspan but then say your concern is due to seismic (plastic hinges near supports). Also you list a load combination of "1.2 DL + 1.6 LL + 1.4 Earthquake + 1 Wind" which does not exist (that I have seen). Then say that for seismic the load combination is 2 x (1.2DL + 1.6LL) which I've also never seen. Then you want to use a steel beam to help carry gravity forces which you've already established that the beam is ok. So....
As best as possible try to organize your concerns with your suggested solutions and lets see if we can resolve this.

EIT

http://www.HowToEngineer.com

 

[releky]

The designer (they have a dozen designers in the company) said they use 23 kinds of load combinations. So all available load combinations of all countries they put this in their ETABS. The 1.2 DL + 1.6 LL + 1.4 Earthquake came from the following:

http://imagizer.imageshack.us/v2/800x600q90/842/ul0u.jpg

It's in page 732 of the book Reinforced Concrete (A Fundamental Approach) by Edward Nawy 5th edition. I asked the designer why his Vu is so high. He said he also used Wind in addition to Earthquark. Next week I'll try to make him produce what is the biggest load combinations out of the 23 he used to give such a big shear *almost* equivalent to 2 x (1.2 DL + 1.6 LL).

After doing some rough manual computation with him (he doesn't know how to manually compute). I realized the stirrups in the constructed concrete is only good enough for 1.2 DL + 1.6 LL (the exact computations I shared a few messages above).

Now I realized that it is the seismic shear the building is deficient where it can't take on the probable moment strength. I discussed this with the designer and he seemed to realize it and agree I-Beam is needed.

See the logic in the above. So I'm now organized what to discuss with him. Of course it is ultimately the design of the designer I'll follow because I'm of course not authorized to design it my own... because he is the designer. And just giving the designers ideas to think about. I'd discuss with 3 of them next week to get my arguments across the shear is deficient for probable moment strength. The website of the designer is

http://rbsanchez.net/

for reference.

 

[AELLC]

OK, I now understand the beams are deep enough, there is no need for deflection checks of cracked section.
You are saying the stirrups are OK for gravity but deficient for seismic.

The part I still don't understand - if you can do manual calculations and who you call the designers cannot - why can't you do a complete design as a "go-by", an example of how a typical building should be designed?

A second thing I still don't understand - if these designers are University graduates, why don't they comprehend shear and moment diagrams? That is very basic, something you don't forget even after years of using "black box" software. I have forgotten many things that were studied in University that I have never used at work, but shear and moment are never forgotten.

Perhaps you mean the designers think that the selection of longitudinal reinforcement is of primary importance, and the stirrups are of much less importance. I have reviewed calculations were the designer seems to think beams are very important, and columns and especially footings are much less important - the calculations for those were sloppy and hurried, and full of deficiencies.

 

[rb1957]

"In the Philippines.. engineers are free from liability in case building is damaged by any seismic activity so designers don't worry about damages caused by earthquake because they can always blame it on the earthquakes. So in a country where 99% of buildings are special moment frames. They are just ordinary moment frames or even just gravity frames."

"You are saying the stirrups are OK for gravity but deficient for seismic." ...

this is the bit that i (in my ignorance) don't understand ... i read the first sentence to say "seismic loads aren't important" and the 2nd "the structure is failing due to seismic loads" ?

Quando Omni Flunkus Moritati

 

[releky]

Let me clarify. When earthquakes occurred to badly design building, the engineers will blame it on the earthquake and be free from reliability even if they design it badly! That is, some don't take responsibility and blame everything on act of god for any uneventful happening. This is the context they don't worry about it.. because they are free from liability when earthquakes strikes.. because we don't have a committee that investigates how the earthquake damage the buildings.. all earthquakes damaged buildings are categorized as act of god in the court of law. This is why contractors do very poor quality works. And city hall engineers can be bribed easily with $100 to bypass any bad design submitted.

In the second sentence. Some of us are very concerned about structures that fail during seismic load.. hence we need absolute quality control check of the design and the execution. And that's what I'm doing.

 

[RFreund]

Well not a bulleted summary but I'll take it.

So we are concerned with shear for seismic...
The 1.4xE is based on using a service level seismic force so be careful as ASCE7 now uses strength level loads so if you apply the 1.4 to the strength level load then you are factoring up this force twice.

Also (and you may already have down this) but the shear capacity at each location along the beam should be compared to the shear demand of the load combination. Meaning the shear diagram with lateral forces applied will be different than with only gravity forces.

Sounds like meeting with the designers should get everyone on the same page... hopefully. Let us know how it goes.

EIT

http://www.HowToEngineer.com

 

[AELLC]

One thing no one has mentioned yet is the detailing of rebar at the column-beam area. I am not familiar with the latest seismic detailing, does anyone have some info?

 

[BAretired]

Not me AELLC, but if the steel beam is expected to help with seismic loading, its connection to the columns is going to be a challenge.

BA

 

[releky]

The problem now can be simply categorized as seismic retrofitting of gravity load structures.

If the steel I-beam underneath the concrete girder is difficult to be moment connected to the column and connected to the beam as composite. Does it make sense if you will retrofit those gravity load structures to use steel I-beam simply to support the concrete beam in the event the shear cracks simply make the beam fall down? So the I-beam would be sectioned just to carry the weight of the beam?

In seismic engineering, the idea of plastic hinging is to act like fuse so after the hinges form, you just repair the beam... and the basic idea is not to let the structures fall but just damaged enough to be repaired. So the I-beam would just to make sure the beam won't fall down and give time for occupants to run safely outside and then repair the damages afterwards (in rare seismic event that may happen once a century or twice or never at all.. seismic engineering is about probability as seismic event can't be predicted).